1. Field of the Invention
The present invention relates to the design of the phase plug in an electro-acoustic transducer composed of the phase plug and drive-unit, and waveguide assemblies.
2. Description of Prior Art
In high performance audio playback systems it is common practice to use wave guiding devices coupled to an electro-acoustic transducer drive-unit (that portion of an electro-acoustic transducer that does the direct electrical to acoustical transduction) that act to both amplify and direct the sound energy created therein. These devices go by many names, the most common being “horns” and “waveguides”, waveguides—a newer term—implies that more importance is given to the directivity controlling aspects of these devices than the loading ones. I will use the term waveguide consistent with its new meaning, but it is also my intention to mean any and all devices performing such tasks regardless of name.
The “shape” of the wavefront presented to the input aperture of the waveguide (the throat) has a great influence on how the device radiates sound from the open end (the mouth), particularly at the higher frequencies. Shape in the context of this application means the bounding area of a plane perpendicular to the sound channel propagation, it is a two dimensional surface, but it can also refer to the surface velocity distribution (complex number map) in that same surface. In order to control the formation of the throat wavefront it is common practice to place a plug containing multiple phase equalizing channels between the drive-unit diaphragm and the throat of the waveguide. This plug is usually designed so as to direct the sound waves emitted from the drive-unit diaphragm into the waveguide throat aperture in a prescribed shape. This plug is also know by various names, the most common being “phase plug”, “phasing plug” or “equalizer”.
The art of phase plug design is well advanced. The first significant disclosure of the function of a phase plug is provided by Wente, U.S. Pat. No. 2,037,187 (1936). Wente notes that the goal is to provide an essentially planar wavefront at the horn throat and he achieves this task by directing the sound through phase equalizing channels of essentially equal length, thereby ensuring that the contributions all add in phase.
In Henricksen, U.S. Pat. No. 4,050,541 (1977), discloses an alternate type of phase plug wherein the phase equalizing channels are radial, as opposed to more common circumferential.
Carlson, U.S. Pat. No. 4,718,517 (1988), discloses a phase plug which has a rectangular exit aperture at the throat end to better match the waveguide throat, but at the input end at the drive-unit diaphragm, the phase equalizing channels are also rectangular which is not ideal. Carlson recognizes the need for a phase plug when transitioning from a circular source to a non-circular one but does so utilizing only a single channel of non-varying cross section (obviously easy to fabricate) or he uses two phase equalizing channels which are still not ideally matched to the circular diaphragm. In the later case the phase equalizing channels vary only in size—not shape—as they progress, and they are not concentric. In terms of performance (but not necessarily so in terms of manufacturing case) Carlson's design is not optimal. The phase plug's phase equalizing channels entrance shape is not well matched to the diaphragm resulting in unequal phase propagation summation effects at the waveguide throat which will result in distortions of the sound.
Bie, U.S. Pat. No. 5,117,462 (1992), utilizes the voice coil motion as part of the radiating surface of the drive-unit and adds a sound channel to said radiating region. He also shows a phase plug with non-concentric round phase equalizing channels.
Finally, Keith, U.S. Pat. No. 6,064,745 (2000), discloses an improved radial type of phase plug which is easier to manufacture than previous designs. The input shapes are not circles, they are triangular.
Each of these inventions claims a distinct improvement over the others in regard to performance, manufacturability and/or some other comparable; however, none of them discloses a phase plug design which optimally matches a drive-unit's diaphragm shape to a waveguide throat of different shape (as can be required by waveguide theory). By using phase equalizing channels which transition gradually from circular at the diaphragm to non-circular at the exit an improved phase plug design can be implemented.
Adamson acknowledged in U.S. Pat. No. 4,975,965 (column 5, lines 56–64) that his phase plug “has been found to be particularly useful when applied to acoustic waveguide speakers of the variety developed by Dr. E. R. Geddes . . . ”—the current inventor. The waveguide referred to by Adamson was based on the oblate spheroidal coordinate system. In Chapter 6 of my book Audio Transducers, (GedLee Publishing, July, 2002), I discuss the fact that there are several different coordinate systems which yield useful waveguides (see table 6.1 in the text) and that each one has a different set of radiation characteristics. As discussed in this text, these different waveguides require different wavefront shapes at the throat (“source aperture” and “curvature” in the table of my text) for optimum performance.
In order to optimally match a drive-unit to a waveguide, it is desirable to have a phase plug that adapts the motion of a generally circular diaphragm to a desired aperture shape in a manner which brings the velocities of various portions of the circular diaphragm to the exit aperture in a prescribed, but generally in-phase, manner. There are situations where the velocity distribution (the velocity shape) in the phase plug's exit aperture (the waveguides throat) may want to be non-uniform.
An preferred embodiment of the invention disclosed in this specification occurs when connecting a circular drive-unit to a waveguide that requires an elliptical throat aperture. The phase plug in this case should have a shape at the input end that is circular or annular such as concentric annular rings spaced so that each ring has the smallest path length to those diaphragm points which excite it, i.e. the shapes conform to the circular source. At the exit aperture the shapes are concentric elliptical rings, typically of equal area and arranged so that there combined shapes equal the shape required by the waveguides throat.
In none of the prior art is it recognized that the optimum phase plug would have the requirements highlighted above. It was generally assumed that the phase plug's exit aperture (the throat of the waveguide) should be circular, or one with an equally simple shape to fabricate so long as the shapes of the sound channels at the input and output ends remain constant. In most cases the waveguide throat is adapted to match the phase plug's exit aperture—not the other way around. The prior art generally conforms to a design approach where ease of manufacture is emphasized, steering away from more complicated sound channel shapes because of manufacturing concerns. Today, injection molding complex phase plugs in plastic is a relatively easy manufacturing process and so complex shapes need no be avoided and more effective phase plugs can be readily fabricated.